The learning curve is different for a snowboard, it's hell the first 2 days as it's all based on balance and without balance you'll just fall. With skis you can basically go down most slopes the first day just by plowing and advancing from there. This is not going to work with a snowboard, you can't really take the lift to the top of the hill and proceed learning on your way down.
In my experience most people learn the snowboard after 3-4 days, but it takes a bit more to feel confident.
For me, I was just an average skier but I felt much more confident on the board than on the skis after the first week or maybe second week. So in my experience the learning curve for the board was slower at first but after a week I increased in skill much faster than I had on the skis previously.
I dislike snowboarding the less steep "transportation" routes though, they are usually narrow, heavily bumpy and you have skiers to the right and left of you who don't understand that on a snowboard you can't speed check or fix your balance without turning the board..
I'm very good with skiing and started with age 35 to snowboard. I learned the basics quick, but now even some years later, I'm pretty good on the board, I just don't have the same safe feeling like on my skies. Now on perfect slopes, I prefer my (slalom) ski, if the weather is a bit snowy or the snow is not perfect, I prefer the snowboard.
I actually tried to get back to skis last year, rented a pair and went up on the red slope, but damn I had forgotten how to do it :) Felt so strange, it was definitely not any "like riding a bike" feeling of motor memory. I guess my cache was flushed. Kind of froze on the slope and couldn't get myself to go down :)
Eventually I did and I didn't fall but it was really scary. I think I need to try again for a day on a green or blue slope...
You can get to a point within a couple of days where you feel comfy falling leaf-ing down most slopes in the mountain. Some terrain, like glades, will always be a bit harder, in my experience, though I'm so comfortable skiing through glades that it might just be a personal thing.
Unless you really devote active learning effort towards it, you could easily remain mediocre at toe-edge and real carving for months. I do both, and I still feel like my skiing is miles ahead of my snowboarding because I've just never gelled with facing up the mountain half the time. YMMV though.
Not sure if that answer is in any way helpful, but I learned to ski as a kid (like the typical kids skiing course every winter, progressing one level at a time) and did one week of snowboarding when I was 15 and then not much more. Then life kinda happened and I'm 32 or 33 and while I had forgotten most things about skiing (and it hurt my legs) but snowboarding (on my basic bad? level) has been fine since then.
I guess I am not your average skier. That said, I have had a snowboard now for over a decade and there is no way I am going to (ever) snowboard the most challenging terrain I can ski. To be fair, I easily snowboard way over 90% of the terrain I ski.
